This invention relates to a device used in spectrometry for exciting a chemical element to emit electromagnetic radiation.
Such a device, commonly known as a plasma burner or torch, is employed in ICP emission spectrometry for detecting elements such as boron, iron, magnesium, phosphorous and zinc. A plasma burner or torch generally comprises an elongate capillary tube, an inner jacket surrounding the capillary tube coaxially therewith, an outer jacket surrounding the inner jacket and coaxial therewith and an induction coil for generating a plasma. A cooling gas is fed to a space between the inner jacket and the outer jacket, while a plasma gas is conveyed to a space between the inner jacket and the capillary tube, the capillary tube serving to guide into the plasma burner an aerosol consisting of a carrier gas and particles of a chemical element to be analysed. A standard plasma burner, as described by S. Greenfield et al., Analyst, Vol. 89, pages 716 to 720 (1964) operates with nitrogen as a cooling gas and argon as the plasma gas. The plasma is generated in the interior of the burner by an induction coil having two or three turns closely surrounding the outer jacket of the burner. A high voltage alternating current applied to the coil generates in the burner a changing magnetic field in turn bringing about an electric ring voltage perpendicular to the axis of the burner. This ring voltage is on the order of 20 to 30 volts and maintains the plasma after ignition has occurred, the plasma taking the form of an elipsoid.
The plasma gas and the cooling gas cause in the flow direction an elongation of the plasma ellipsoid and a considerable flattening thereof in the counterflow direction. The resulting geometry of the plasma ellipsoid enables the aerosol gas stream emanating from the mouth of the capillary tube to pierce the ellipsoid in the center thereof so that it assumes an annular shape. Only in this case is the aerosol gas sufficiently heated to excite the subject chemical element contained in the aerosol sample to emit electromagnetic radiation. The light emission due to the excitation can then be analyzed qualitatively as well as quantitatively by means of a spectrophotometer.
In the event the plasma ellipsoid is not pierced by the aerosol gas, the ellipoid acts like a solid body on the flow of the aerosol, thereby causing a major portion thereof to flow past the plasma unheated.
The pressure distribution in the plasma and the magnetically induced backflow thereof are determined in part by the flow of the plasma gas and of the cooling gas. For example, if the flow of the plasma gas and the cooling gas has a tangential component, the backflow velocity is reduced, which reduction facilitates the introduction of the aerosol gas. It is to be noted that the primary purpose of the cooling gas is to cool the quartz wall which is in contact with the plasma gas.
Argon/nitrogen burners require an argon flow of approximately 5 to 10 liters per minute and a nitrogen gas flow of approximately 15 to 20 liters per minute. The total consumption of gas is considerable. It is 2 to 5 times as large as the plasma gas flow. One method of reducing the gas consumption, particularly the consumption of expensive argon gas, has involved reducing the dimensions of the burner. However, this solution has a negative affect on the stability of the plasma. To control the plasma stability the amount of aerosol gas introduced into the plasma must be decreased, which decrease has the effect of lowering the detection sensitivity or power. In general, the performance of a small plasma burner is not as high as the performance of a standard burner. See A. D. Weiss et al., Analytica Chimica Acta, Vol. 124, pages 245 to 258 (1981). Some prior art plasma burners are provided with water cooling, as described by Guy R. Kornbulm in the article "Reduction of Argon Consumption by a Water-Cooled Torch in Inductively Coupled Plasma Emission Spectrometry, "Analytical Chemistry, Vol. 51, No. 14, December 1979, pages 2378 to 2381, and by H. Kawaguichi in the article "Water-Cooled Torch for Inductively Coupled Plasma Emission Spectrometry," Analytical Chemistry, Vol 52, 1980, pages 2440 to 2442.
In such plasma burners or torches, the water implemented cooling of the outer wall can reduce gas consumption substantially. However, the flow of cooling gas cannot be eliminated altogether because otherwise the flow and pressure conditions of the plasma are detrimentally altered.
An object of the present invention is to provide an improved plasma burner or torch of the above described type in which gas consumption is markedly reduced, thereby lowering the high operating costs of ICP emission spectrometry.
An additional object of the present invention is to provide such a plasma burner which retains a high level of performance and analytical capability.